Surge is a well-known condition which is to be avoided in the operation of compressors. An efficiently operated compressor is designed to be operated as close to the surge line as possible in order to obtain maximum efficiency. In a compressor, surge occurs when the volume of gas delivered to the inlet of the compressor falls below a predetermined limit. If the volume of gas is below the predetermined limit, the compressor will run out of gas to compress resulting in a reverse flow from the high pressure output to the low pressure input. Failure to reduce the resistance to the flow of gas through the compressor causes the surging cycle to repeat until a correct volume of gas flows through the compressor from the inlet to the outlet.
A conventional solution for controlling surge in a centrifugal compressor is to provide a bypass valve which couples compressed gas from the outlet back to the inlet which functions to provide sufficient mass flow of gas to the inlet to prevent the surging condition. Various control schemes exist for controlling the flow rate of gas through a surge control valve in a bypass line between a point of higher pressure in a compressor and a point of lower pressure to control surge.
U.S. Pat. No. 2,969,805 discloses a surge controller which uses pneumatic control of a relief valve to prevent surge. The pneumatic control is complex in structure, is not easily integrated into electronic control systems for valves which are in use today and does not have the reliability of electrical systems. The pneumatic control detects sudden positive or negative pressure variations on the output of the compressor as the control parameter for the relief valve. However, the '805 patent does not disclose a surge control for a multiple stage compressor and does not disclose detecting the amplitude of pressure variations at each stage of compression and controlling the relief valve with a largest amplitude pressure variation detected at all of the stages. The '805 patent does not disclose that any particular frequency range of pressure variation is optimal for controlling the relief valve.
U.S. Pat. No. 4,156,578 discloses the control of surge by complex electrical signal processing as a function of several variables of system operation. The complexity of this system is a disadvantage in applications such as an airframe vapor cycle cooling system.
U.S. Pat. No. 4,807,150 discloses a surge control which minimizes the recycling of compressed gas between an inlet and an outlet stage. The system utilizes electrical signal processing to incrementally close the control valve toward a fully closed position until the flow is close to the surge limit or until the actual deviation of the set point flow in the actual process flow exceeds a desired high limit for the deviation. This system is complex which makes it undesirable for applications as an airframe vapor cycle cooling system.